Typically, an integrated circuit (IC) on a semiconductor die includes an array of bond pads arranged surrounding the area of the die occupied by components of the IC. The bond pads provide contact areas for wirebonding of leadwires to provide electrical I/O connections from the IC to terminals of an IC package. For example, a high lead count IC may have 256 contact pads arranged around the four sides of a square die for assembly into a package such as a Quad Flat Pack (QFP). As device geometries diminish and integrated circuits become more complex, a larger number of I/O connections are required. The minimum die size for an IC design may be determined primarily by the area taken up by the bond pads, i.e. by the number of bond pads and their arrangement, rather than the dimensions of other components of the integrated circuit, and the minimum die size is said to be "padlimited".
Using known wirebonding apparatus, with a minimum contact pad size of typically .about.5 mil (.about.125 .mu.m) and a minimum pad pitch typically in the range of 6.5 to 10 mil (165 .mu.m to 250 .mu.m), 256 contacts can be accommodated with a die size of .about.500.times.500 mils (.about.12.times.12 mm), yielding about 80 dice per 6 inch wafer. By reducing the area taken up by the bonding pads on each die, the number of dice per wafer, and therefore wafer yield, is increased. However, there is a practical limit to reduction of bond pad size and bond pad pitch while maintaining high yield wirebonding capability with conventional wirebonding apparatus.
The minimum bond pad size and the minimum bond pad pitch are primarily determined by the physical dimensions of the wirebonding tool, which also determines the size and form of the wirebond.
To maintain high output of packages per hour, wirebonding speed is an important consideration for high lead count packages. Processing rates of 6 to 7 wires per second (or .about.150 ms per wirebond) may be achieved with known wirebonding apparatus. Known apparatus provides for relative movement of the die and the wirebonding tool in an xy plane parallel to the plane of the die, and in a z direction towards and away from one another, so that the tool may be stepped from one bond pad location to the corresponding individual leadframe contact area to form a first leadwire and then back to an adjacent bond pad to form an adjacent leadwire extending to a corresponding adjacent leadframe contact area. Each wirebond is made sequentially around the die in a continuous loop from one bond pad to an adjacent bond pad, so as to minimize the total distance travelled and thus reduce the total wirebonding time.
Generally, contact areas on leadframe fingers of a conventional IC package are spaced apart a greater distance than the corresponding bond pad pitch of the die In a packaged die, the resulting arrangement of bonded leadwires diverge, or fan out, from the bond pads on the die towards the lead fingers, with an increasing fan out angle towards the corners of the die package. Thus, in general, leadwires extending near the centre line of each side of a rectangular die extend nearly perpendicularly from an axis defining a row of contact pads, but wires extending from pads near the corners fan out at angles approaching .about.45.degree. or more relative to the leadwires near the centreline.
Consequently, towards the corners of the die, bonded leadwires may extend towards or even extend across neighbouring pond pads. Thus, the minimum bond pad pitch should be sufficiently large to avoid undesirable electrical contact between a bonded leadwire and a neighbouring leadwire and bond pads. However, more significantly, if the bond pad pitch is not sufficiently large, access for the wirebonding tool to provide a wirebond on a bond pad adjacent an already bonded leadwire is restricted by an already bonded leadwire extending across the adjacent bond pad. In fine pitch wirebonding, problems due to wirebonding tool-to-wire interference may arise if the bond pad pitch is reduced. In known methods of wirebonding, the latter problem is most severe when wirebonding leadwires with large fan out angles, near corners of a die.
The shape and form of the wirebonding tool and the maximum diameter of the tool extending between adjacent wirebonds are thus important parameters in determining the minimum bond pad pitch. Conventional wirebonding tools for example as used for gold ball bonding, use capillaries having a simple 30.degree. conical tapered end. In a known approach to reducing wire-to-tool interference for fine pitch bonding, there is now available a type of wirebonding tool called bottleneck capillary tool, which describes the form of the tapered end which narrows from a broader shoulder to a narrower end of a tubular conical form having a .about.10.degree. taper. A bottleneck capillary tool, with its less tapered end and smaller maximum diameter, can approach a neighbouring leadwirebond more closely than a conventional tapered capillary. Thus the minimum bond pitch which can be made without damaging an adjacent wirebond is reduced. Theoretically, based on purely geometrical considerations, an available capillary wirebonding tool, for example, K&S 41490-Bottleneck Tailless Thermosonic Capillary for use with 1 mil gold wire, may be centred only 4.5 mil (112 .mu.m) from an adjacent bonded leadwire. However, a finite clearance between an already bonded leadwire and the capillary bonding tool is necessary to prevent breakage of previously bonded wires or damage to the delicate fine tip capillary bonding tool.
Furthermore, during wirebonding using known wirebonding apparatus, there is a statistical variation in placement of a bond on a pad. Ideally, each bond is o centred on a bond pad, but there is a limitation on the machine and operator accuracy for known wirebonding apparatus which may result in a variation of as much as +/-20 .mu.m and typically +/-10 .mu.m, in placing a bond on a pad. Consequently, the practical limit on the minimum bond pad pitch is greater than that calculated from purely geometrical considerations, i.e. based on the dimensions of the wirebonding tool and the loop profile of a bonded leadwire. A practicably achievable minimum bond pad pitch must take into account factors including wirebonding tool geometry, and placement accuracy of the wirebonding apparatus. Thus, with conventional wirebonding apparatus, wire-to-tool interference is a significant limitation to reduction of the bond pad pitch, particularly for leadwires with large fan out angles, near the corners of a die.
In a method as described in copending U.S. patent application Ser. No. 07/957,955 filed on Oct. 8, 1992 to Simpson, entitled "Semiconductor Die With Variable Bond Pad Pitch", a die is provided with variable bond pad spacing, i.e. the spacing of the bond pads is a minimum at the centre of each side of a die for wires with a fan out angle of zero where tool-to-wire interference is minimized and the bond pad pitch increases towards corners of a die with increasing fan out angle. Thus the dimensions of a die may be reduced, while ensuring for wirebonding leadwires with larger fan out angles.
However, for conventional wirebonding apparatus, wire-to tool interference remains a significant limitation on the reduction of bond pad pitch, particularly for leadwires with large fan out angles near the corners of a die.